Also, I would advice to migrate only clients you created, or at least modified. Don't migrate pre-packaged/system clients, like oxTrust's and IDP's clients, or SCIM or UMA related clients. I'm pretty sure that at least oxTrust's and IDP clients are referenced from other parts of configuration, so you would need to do a few more surgical updates, which are not covered by Community Support
Also it's quite possible some small changes to layout of client's LDAP entry were introduced between 4.1 and 4.2, which may make them incompatible, so you may need to do a attribute to attribute comparison to figure this out. Excluding pre-packaged clients in this case will help you to at least keep access to admin console.